Biology Reference
In-Depth Information
animals such as cows and goats had been domesticated, and thus where a supply of
milk was available to adults. This mutation spread throughout Northern Europe by
positive natural selection because it conferred survival and reproductive advantages
on those who carried it. Today this mutation occurs in 95-98% of Europeans, but in
only 20-50% of Hispanics and 5% of Asians. Adult people who lack this mutation
suffer from the condition called “lactose intolerance” because the inability to digest
lactose results in bloating and cramping.
It is the few mutations that have positive effects that provide the variation whose
selection drives the evolutionary process. So we come to the surprising realisation
that the diversity of life is created by mistakes - errors made when DNA is copied.
If mutations did not occur, evolution would not be possible.
Evolution is the result
of a series of successful mistakes.
So the conclusion that biologists have reached is that, because both mutation
and natural selection are observable facts
,
evolution is inevitable.
Thus evolution is
both a theory and a fact.
But evolution is more than just another scientific theory
because it challenges those views that suggest humans are basically different from
other animals and so can escape the laws of nature. It is this aspect of evolution
that makes it so unattractive to many people. But rejecting evolution means that we
reject the best means we have found so far to understand ourselves and our place in
the world.
Hierarchical Classification
We humans have a strong desire to order objects into categories because this helps
us to make sense of the world. Before experimental biology got underway at the
end of the nineteenth century, most biologists were occupied in trying to clas-
sify all the living organisms they could find into groups. The characters they used
for this purpose were morphological ones, but today we have, in addition, the
sequences of DNA and proteins. All organisms can be classified into groups, using
the system promoted by the Swedish biologist Carolus Linnaeus in the eighteenth
century. Linnaeus used five groups but today this has expanded to eight groups,
called domains, kingdoms, classes, orders, phyla, families, genera and species.
These groups are defined by agreed sets of characteristics. For example, the king-
dom of metazoa, or animals, is defined as those multicellular eukaryotes that eat
other organisms, not including organisms such as
Darlingtonia
that are obviously
plants. It is important to appreciate that of these eight groups, only the species has
a real existence - all the other groups are constructs of the human imagination that
selects certain characters as being more useful for classification purposes than oth-
ers. Species can be said to have an existence independent of how humans view them
because the most commonly used definition of a species is a group of interbreeding
individuals.
Each of these eight groups is defined by an agreed set of similar characteris-
tics, but the important feature of this scheme for the point of view of evolutionary
theory is that these sets are nested within one another. This nested pattern of groups
